Author(s): Davide Vettori; Roberta Davello; Riccardo Vesipa; Costantino Manes; Luca Ridolfi

Linked Author(s): Davide Vettori, Costantino Manes

Keywords: Escherichia coli; Bacteria inactivation; Hydrodynamic stresses; Chlorination

Abstract: Water disinfection in treatment plants heavily relies on the use of chemicals to prevent microbiota from entering the water supply system. An unintended consequence of using chemicals is the creation of disinfection by-products that contaminate drinking water and surface-waters, impacting water ecosystems and increasing the risk of chronic illnesses for humans and animals alike. For this reason, water utilities aim to reduce the use of chemicals to simultaneously meet water disinfection targets and prevent water toxicity due to by-products by employing combinations of techniques and/or by switching to more environmentally friendly techniques (e.g. UV). Stemming from recent promising results of hydrodynamic cavitation as low-cost alternative disinfection technique, with this work we aim to assess the effects of hydrodynamic stresses associated with different flow regimes on the inactivation of Gram-negative bacteria Escherichia coli. We present results from two sets of experiments: (i) in laminar flows within a rotational rheometer to identify a critical level of shear stress over which bacteria are inactivated; and (ii) in turbulent flows in a pilot plant to assess the disinfection efficiency of chlorination combined with a strong turbulent regime. The results of experiments in laminar flows show that E. coli remain viable at steady shear stresses up to 4240 Pa applied for many minutes, contrary to previous findings. Experiments in the pilot plant indicate that the presence of a strong turbulent regime during chlorination magnifies both the bacterial removal efficiency and the bacterial removal rate of sodium hypochlorite. Moreover, bacteria that are exposed to a turbulent flow prior to chlorination display a higher inactivation, with removal efficiency up to 100% at concentration of sodium hypochlorite of 0.05 mg/l (which is 75% lower than the standard concentration used in water treatment plants), suggesting that bacteria may have a ‘plastic’ response to the flow conditions they are exposed to. By interpreting our results in light of previous works with E. coli in ‘simple’ flows, the existing knowledge on modelling cell dynamics, and the mechanisms of bacterial inactivation by chemicals, we conclude that turbulence has a critical role in bacteria inactivation. In more detail, we provide evidence that high levels of hydrodynamic stresses are not sufficient to cause E. coli but turbulence makes bacteria more sensitive to the effects of sodium hypochlorite likely because of sublethal damages caused by strong variations in hydrodynamic stresses. Hence, combining chlorination (or other chemical treatments) with a controlled turbulent regime can greatly improve the disinfection efficiency by reducing the concentration of chemicals and contact time required for disinfection.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221244

Year: 2022